Implantable medical devices for producing a therapeutic result in a patient are well known. Examples of such implantable medical devices include implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators, cochlear implants, and others that now exist or may exist in the future. These devices are intended to provide a patient with a therapeutic output to alleviate or assist with a variety of conditions. Typically such devices are implanted in a patient and provide a therapeutic output under specified conditions on a recurring basis.
One type of implantable medical device is a drug infusion device which can deliver a medication, typically fluid medication, to a patient at a selected site. A drug infusion device may be implanted at a location in the body of a patient and deliver a fluid medication through a catheter to a selected delivery site in the body. Examples of such devices are described in U.S. Pat. No. 5,782,798 to Rise, entitled Techniques for Treating Eating Disorders by Brain Stimulation and Drug Infusion; U.S. Pat. No. 5,814,014 to Elsberry et al., entitled Techniques of Treating Neurodegenerative Disorders by Brain Infusion, each assigned to Medtronic, Inc., Minneapolis, Minn.
Another type of implantable medical device is an electrical stimulation device. An electrical stimulator can also be implanted in the body of a patient and can stimulate selected nerves in the body in accordance with a specified routine. The electrical stimulator may be implanted at a location in the body and deliver electrical stimulation pulses through a lead or leads to a stimulus site. One example of such an implantable electrical stimulation device is Medtronic's RestoreUltra™ neurostimulator.
It is desirable to be able to non-invasively program an implanted medical device, such as a drug infusion device or an electrical stimulation device, in order to change the therapeutic regimen without incurring unnecessary trauma to the patient. An example of such a device is described in U.S. Pat. No. 4,692,147 to Duggan, entitled Drug Administration Device, assigned to Medtronic, Inc., which can be non-invasively programmed to change both the dosage amount and the dosage interval. Verification of the received dosage and interval commands are achieved by means of an audio transducer which is attached to the device case. The implantable drug administration device described in Duggan allows a medical professional to program the delivery rate of a drug contained in the reservoir of the device over a specified interval. The process, however, may be labor intensive.
Non-invasively programmable implantable medical devices are typically programmed using an external programming device, sometimes known as a controller, which can communicate with the implanted medical device through well known techniques such as telemetry. An external controller, or programmer, can be used by a medical professional, for example, to change the therapeutic regimen by increasing or decreasing the amount of fluid medication delivered or by increasing or decreasing the intensity or timing or characteristic of an electrical stimulation signal. Typically, a medical professional interfaces with the external controller or programmer to set various parameters associated with the implantable medical device and then transmits, or downloads, those parameters to the implanted medical device. The external device may also record other information important to the delivery of the therapeutic output although not actually downloaded to the implanted medical device, e.g., patient information, implanted device information such as model, volume, implant location, length of catheter or lead, and other information specific to different devices.
The implementation of a therapeutic program requires timing to be very precise. It has been found that timers may include some amount of time drift. Drifts on the order of +/−15 minutes per year may not be unusual, causing programmed events, such as a bolus from an implantable drug pump, to be off by as much as 15 minutes, in a given year, and up to 105 minutes in pump's typical 7 year life if left uncorrected. In normal drug pump procedures, for example, the pump does not go more than approximately 180 days without being filled. The drift over this time period therefore could be on the order of seven to eight minutes. As may be appreciated, depending on the type of implantable device, the specific programming, and other factors, may make the clock drift more or less significant. However, a system of updating the correct clock time to the drug pump is therefore needed.